1. Field of the Art
This invention relates to an apparatus for holographic interferometry particularly suitable for use in measuring surface configurations of cylindrical optical surfaces of such as cylindrical lenses and cylindrical mirrors or of a product with a similarly cylindrical surface to be inspected (hereinafter referred to simply as "cylindrical specimen" or "specimen lens" for brevity).
2. Prior Art
Interferometers are widely resorted to, for example, for non-contacting precision measurement of surface configurations of optical elements. Typical of the interferometers usually employed for inspection of specimens of the above-mentioned nature is the so-called Fizeau's interferometer, which employs laser light as a light source and which is arranged to reflect part of the light rays from the laser light source on a surface of a reference lens, while reflecting on a surface of a specimen lens part of light rays transmitted through the reference lens and measuring the resulting interference fringes of the two light reflections. More specifically, a laser beam from the laser light source is reflected on (or passed through) a beam splitter and then collimated into parallel light rays through a collimator lens, projecting the parallel light rays toward the specimen lens through the reference lens, transmitting through (or reflecting on) the beam splitter the two light reflections from the light incident surfaces of the reference lens and the specimen lens, and focusing an image of interference fringes, which is produced by interference between the two light reflections, at a predetermined position through an interference fringe imaging lens for observation purposes. In case the specimen lens is a spherical lens, an accurate measurement is infeasible unless the center of curvature of the surface of the specimen lens under inspection is held exactly in alignment with the center of curvature of the reference surface of the reference lens.
Recently, with the advent of technology for computerized fabrication of holographic patterns, there has been a trend toward using a holographic interferometer in the measurement of surface conditions of various specimens to be inspected. The holograms obtained by computerized fabrication processes are usually in the form of an optical element which is formed by coating a photoresist film on a substrate of glass, exposing the photoresist film to a holographic pattern, e.g., a grating pattern equivalent to the interference fringes of object and reference waves, by scanning the photoresist film with an electron beam, and developing the holographic pattern to manifest the interference fringes. This holographic optical element is designed to be able to reproduce the object wave upon casting on the interference fringes a reproduced wave equivalent to the reference wave. The holographic interferometry, which is useful for inspection or measurement of special surface configurations, is expected to have a wide range of applications in the future.
A large number of cylindrical lenses or other cylindrical optical elements are used on copier machines, facsimile machines or other apparatus which incorporate an optical scanning system. The surface conditions of a finished cylindrical surface on such a cylindrical optical element can be measured or inspected by means of a holographic interferometry device using a holographic optical element with a grating pattern containing rectilinear stripes or lines in parallel relation with the generator of the cylindrical surface under inspection. Even in case of inspection of a cylindrical optical element, correct measurement is possible only when it is located in correct direction and position relative to the holographic optical element. Unlike spherical lenses, the surface of a cylindrical optical element which has an arcuate surface configuration in a direction perpendicular to the direction of the generator possesses a uniform shape in the direction of the generator, necessitating to provide an extremely complicate positioning mechanism therefor. In addition, the positioning accuracy has to be controlled to an extremely fine level in order to ensure accurate measurement. However, there has not been developed any interferometric apparatus with a precision positioning mechanism for positioning a cylindrical specimen in a predetermined position on a specimen holder table with strictly accurate directional orientation. Naturally, in step with the increasing applications of cylindrical optical elements, there has been a growing demand for an interferometric apparatus which is capable of measuring the surface conditions of such cylindrical optical elements with a high degree of accuracy.